The present disclosure relates to agriculture. The teachings thereof may be embodied in methods to determine a percentage of weeds and/or control device. For example, a method for determining a weed percentage in an observation section of a field may include: acquiring actual spectral information using an optical observation section sensor aimed at the observation section; acquiring reference spectral information using an optical reference sensor aimed at a reference section of the same ground area; determining a difference between the actual spectral information and the reference spectral information; and mapping the difference to the weed percentage using predefined mapping.